1. Field of the Invention
The present invention relates in general to a shock absorber, and more particularly to a shock absorber including a damping force control device, in particular, a damping force control device using leaf valves.
2. Discussion of the Related Art
A shock absorber including a damping force control device is known. An example of this type of shock absorber is disposed on page 15, second edition of "Instruction Manual for New Model TOYOTA SOARER" published on Aug. 2, 1996 by Servicing Department, Toyota Jidosha Yabushiki, Kaisha, the assignee of this application. The shock absorber disclosed in this Instruction Manual includes (a) a cylinder body, (b) a piston received in the cylinder body and cooperating with the cylinder body to define tow chambers on the opposite sides of the piston, one and the other of the two chambers respectively functioning as a high-pressure chamber and a low-pressure chamber in which a pressure of a working fluid is lower than that in the high-pressure chamber, (c) a piston rod fixed to the piston, and (d) a damping force control device including (i) a hard valve which permits a flow of the working fluid therethrough from the high-pressure chamber toward the low-pressure chamber when a difference between the pressures in the high-pressure and low-pressure chambers is larger than a predetermined threshold, (ii) a soft valve which permits the flow of the working fluid therethrough from the high-pressure chamber toward the low-pressure chamber when the above-indicated difference is not larger than the predetermined threshold, and (iii) a variable flow restrictor device for controlling a cross sectional area of fluid flow of a hard-valve by-passing passage which is formed in parallel with the hard valve.
In the shock absorber disclosed in the above-identified Instruction Manual, the variable flow restrictor device is associated with the hard-valve by-passing passage which is provided in parallel with the hard valve, while the soft valve is disposed in series with the variable flow restrictor device and in parallel with the hard valve. While the difference between the pressures of the working fluid in the high-pressure and low-pressure chambers is not larger than the predetermined threshold, namely, while the hard valve is held closed, the working fluid is forced to flow from the high-pressure chamber toward the low-pressure chamber through the soft valve and the hard-valve by-passing passage. When the hard valve is opened, a portion of the working fluid flows through the hard valve toward the low-pressure chamber. A resistance to the flow of the working fluid through the hard-valve by-passing passage decreases as the cross sectional area of fluid flow of the by-passing passage is increased. Accordingly, the damping force generated by the shock absorber decreases with an increase in the cross sectional area of fluid flow of the hard-valve by-passing passage, provided the velocity of movement of the piston relative to the cylinder body (i.e., the operating speed of the shock absorber) is held constant. Thus, the damping force of the shock absorber is controlled by the variable flow restrictor device by controlling the cross sectional are of fluid flow of the hard-valve by-passing passage.
In the shock absorber disclosed in the above-identified Instruction Manual, each of the hard valve and the soft valve includes an elongation valve and a contraction valve. The elongation valve permits a flow of the fluid therethrough from the high-pressure chamber toward the low-pressure chamber only when an assembly consisting of the cylinder body and the piston rod is elongated with a relative movement of the piston rod and the cylinder body in the direction away from each other. The contraction valve permits a flow of the fluid therethrough from the high-pressure chamber toward the low-pressure chamber when the assembly of the cylinder body and the piston rod is contracted with a relative movement of the piston rod and the cylinder body in the direction toward each other. The provision of the elongation and contraction valves for each of the hard and soft valves results in an increase in the structural complexity of the piston within the cylinder body, and an undesirable increase in the size of the shock absorber.
The above problem is encountered not only where the hard valve is disclosed in parallel with the soft valve and the hard-valve by-passing passage, but also where the hard valve is disposed in parallel with the hard-valve by-passing passage, while the soft valve is disposed in series with a parallel circuit of the hard valve and the hard-valve by-passing passage, and where a soft-valve by-passing passage is provided in parallel with the soft valve.
In the known shock absorber indicated above, the working fluid does not flow from the high-pressure chamber toward the low-pressure chamber, inhibiting a relative movement between the piston rod and the cylinder body while a load acting on the shock absorber is not large enough to open the soft valve, that is, while the soft valve is held closed. When the soft valve is opened, the fluid flows through the soft valve and the hard-valve by-passing passage, permitting the above-indicated relative movement. When the load acting on the shock absorber is large enough to open the hard valve, a portion of the fluid flows through the hard valve. The damping force changes with the operating speed of the shock absorber, as shown by solid line in FIG. 30A.
Although the damping force of the known shock absorber is controlled as described above, this chock absorber is not capable of controlling its damping force, as shown by solid lines in FIGS. 30B and 30C, when the hard valve is open or when neither the soft valve nor the hard valve is open, while the operating speed (relative movement of the piston rod and the cylinder body) is considerably low.
JP-A-62-114239 shows a shock absorber including a valve consisting of two leaf valves superposed on each other. in the direction of their thickness, so as to permit the working fluid to flow from a high-pressure chamber toward a low-pressure chamber. These two leaf valves are both flexed concurrently irrespectively of the operating state of the shock absorber. Accordingly, the shock absorber does not have a sufficient degree of freedom of design, in terms of the damping characteristics, in particular.